As noted in the U.S. patent to R. Rosenberg and P. K. Runge, U.S. Pat. No. 3,766,489, issued Oct. 16, 1973, dye lasers are attractive because of the extremely broad tuning bandwidths that can be achieved. However, in such lasers, it is necessary to focus the pumping light beam to a maximum extent in the dye medium in order to reduce the oscillation threshold. The dye bleaching and other damage which results from such intense pumping is rendered unobjectionable by flowing the dye through the pumping region.
In the cited Rosenberg et al patent a thin free-flowing dye stream was achieved by means of a special nozzle through which the dye solution was discharged. This technique has now been widely adopted. Unfortunately, the instability of the fluid dynamics of the free-flowing stream affects the stability of the output. This instability also limits the choice of solvents based on viscosity. Further, the dye stream cannot be made as thin as one would like.
In the still earlier approach, the dye solution flowed through the focused laser pump spot between glass windows or plates that were permanently positioned (i.e., stationary). However, since the spacing between these windows must, of necessity, be slight, it was difficult to force the dye solution down between them at the desired rate of flow without producing fluid turbulence, window deformations, etc. Also, the concentrated laser pump spot was continually focused at the same place on the windows and this caused burn or heat induced degradations of, and deformations in, the windows. Because of these handicaps, the aforementioned free-flowing dye stream approach was adopted.